Meh. I can see why someone raised in an unusual family (which they are bound to notice at some point, more or less early), would want to have that information.
As much as I wish women could have children without men, this is not the case yet.
And to know your own origin is part of your identity.
Doesn't mean the father has to be a part of the family. Just that this info is available should be enough.
As for the family where the two lesbians have a sperm donor, who has his own family and they are all a big family, I think it's an illusion. I think the guy, deep down, sees it as his harem (it's obviously an American : he has a cap, short pants and is half shaved).